Fix CVE-2018-11806:

https://lists.gnu.org/archive/html/qemu-devel/2018-06/msg01012.html
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2018-11806

Patch copied from upstream source repository:

https://git.qemu.org/?p=qemu.git;a=commitdiff;h=864036e251f54c99d31df124aad7f34f01f5344c

From 864036e251f54c99d31df124aad7f34f01f5344c Mon Sep 17 00:00:00 2001
From: Prasad J Pandit <pjp@fedoraproject.org>
Date: Tue, 5 Jun 2018 23:38:35 +0530
Subject: [PATCH] slirp: correct size computation while concatenating mbuf

While reassembling incoming fragmented datagrams, 'm_cat' routine
extends the 'mbuf' buffer, if it has insufficient room. It computes
a wrong buffer size, which leads to overwriting adjacent heap buffer
area. Correct this size computation in m_cat.

Reported-by: ZDI Disclosures <zdi-disclosures@trendmicro.com>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
---
 slirp/mbuf.c | 11 +++++------
 slirp/mbuf.h |  8 +++-----
 2 files changed, 8 insertions(+), 11 deletions(-)

diff --git a/slirp/mbuf.c b/slirp/mbuf.c
index 5ff24559fd..18cbf759a7 100644
--- a/slirp/mbuf.c
+++ b/slirp/mbuf.c
@@ -138,7 +138,7 @@ m_cat(struct mbuf *m, struct mbuf *n)
 	 * If there's no room, realloc
 	 */
 	if (M_FREEROOM(m) < n->m_len)
-		m_inc(m,m->m_size+MINCSIZE);
+		m_inc(m, m->m_len + n->m_len);
 
 	memcpy(m->m_data+m->m_len, n->m_data, n->m_len);
 	m->m_len += n->m_len;
@@ -147,7 +147,7 @@ m_cat(struct mbuf *m, struct mbuf *n)
 }
 
 
-/* make m size bytes large */
+/* make m 'size' bytes large from m_data */
 void
 m_inc(struct mbuf *m, int size)
 {
@@ -158,12 +158,12 @@ m_inc(struct mbuf *m, int size)
 
         if (m->m_flags & M_EXT) {
 	  datasize = m->m_data - m->m_ext;
-          m->m_ext = g_realloc(m->m_ext, size);
+	  m->m_ext = g_realloc(m->m_ext, size + datasize);
 	  m->m_data = m->m_ext + datasize;
         } else {
 	  char *dat;
 	  datasize = m->m_data - m->m_dat;
-          dat = g_malloc(size);
+	  dat = g_malloc(size + datasize);
 	  memcpy(dat, m->m_dat, m->m_size);
 
 	  m->m_ext = dat;
@@ -171,8 +171,7 @@ m_inc(struct mbuf *m, int size)
 	  m->m_flags |= M_EXT;
         }
 
-        m->m_size = size;
-
+        m->m_size = size + datasize;
 }
 
 
diff --git a/slirp/mbuf.h b/slirp/mbuf.h
index 893601ff9d..33b84485d6 100644
--- a/slirp/mbuf.h
+++ b/slirp/mbuf.h
@@ -33,8 +33,6 @@
 #ifndef MBUF_H
 #define MBUF_H
 
-#define MINCSIZE 4096	/* Amount to increase mbuf if too small */
-
 /*
  * Macros for type conversion
  * mtod(m,t) -	convert mbuf pointer to data pointer of correct type
@@ -72,11 +70,11 @@ struct mbuf {
 	struct	mbuf *m_prevpkt;	/* Flags aren't used in the output queue */
 	int	m_flags;		/* Misc flags */
 
-	int	m_size;			/* Size of data */
+	int	m_size;			/* Size of mbuf, from m_dat or m_ext */
 	struct	socket *m_so;
 
-	caddr_t	m_data;			/* Location of data */
-	int	m_len;			/* Amount of data in this mbuf */
+	caddr_t	m_data;			/* Current location of data */
+	int	m_len;			/* Amount of data in this mbuf, from m_data */
 
 	Slirp *slirp;
 	bool	resolution_requested;
-- 
2.17.1

